In a traveling wave (TW) separation, ions of different mobilities separate based on their relative motion in a moving electric field, typically by the intermittent application of DC voltages. This TW profile moves in the intended direction of ion motion. The TW creates periodic highs and valleys, and the ions are trapped in valleys if the field moves very slowly relative to their mobilities. As the TW moves forward, depending on the speed of motion of the TW, the ions either stay within their valley or roll over the wave to fall back into the previous potential valley. The number of such rollovers is dependent on the ion mobility of the species and this leads to mobility based separation; species with lower mobilities roll over more often and take longer periods to traverse a given distance.
With conventional ion mobility separations, larger voltages are required as the separation distance is increased, as in traditional mobility separations that use constant drift fields. Thus, extremely long path length separations are not feasible. TW based separations can be used to avoid this limitation, however limitations still persist. The practical realization of the benefit of TW ion mobility separations is limited by considerations that include the peak broadening due to diffusion ion roll over in the traveling waves. This results in broad peaks for ion mobility separations when using very long path lengths, making detection difficult and signal-to-noise (S/N) low. Further, multi-pass/cyclical path ion mobility separations are similarly limited in their extent due to peak broadening and signal dilution at large number of passes. Indeed, for such devices one peak can expand by such effects to fill the entire path, and making the approach ineffective for even species of very similar mobility. The solution to this problem for TW separations would enable overcoming diffusional/peak broadening related issues and allow novel instrumentation providing very high ion mobility spectrometry (IMS) resolution.
A related challenge in such application is to increase the initial ion population significantly, so as to increase the S/N at the time of detection, but space charge effects limit the size of the ion population that can initially injected for IMS separations. Thus, while an ion trap is often used to accumulate ions for injection to IMS, a key limitation is the space charge capacity that limits the maximum number of charges, typically to about 106 or at most 107. While the initial injection pulse can be made greater by extending it over a longer period, a longer injection pulse also makes peaks wider and is incompatible with the desired higher resolution. No solution to the problem has been evident, resulting in the need to often repeat the separation many times, and then sum or average the results, to improve the S/N.
In IMS, achieving high resolution has been traditionally addressed by: 1) increasing the physical size of the IMS cell by building a long path length, 2) increasing the pressure, and 3) in a few cases, circulating ion packets in cyclic or multi-pass devices. Increasing the physical size of the IMS cell is hindered by the practicality of fabricating such systems and, in the case of constant field IMS, increasing the physical length requires a proportional increase of the drift voltage. The maximum drift voltage is limited by the electrical breakdown phenomena. Alternatively, the pressure of the buffer gas can be increased which is, however, accompanied by a significant loss of ions due to the poor ability to trap ions at high pressure over extended periods. Increasing the pressure also requires a proportional increase of the drift voltage in constant field IMS which, as mentioned above, is limited by the breakdown voltage. Finally, the path length can be increased by circulating ion packets multiple times in a cyclotron device in order to achieve high resolution. However, the number of passes that can be usefully applied is progressively limited by both the increasing separation between ions and the increased size of the peak, and ultimately as one peak fills the entire device. The length of the cyclic or multi-pass arrangement can be made greater to increase the range of mobilities that can be separated simultaneously, however such devices are cumbersome and difficult to fabricate. As such, there is a need for novel approaches to solve the aforementioned challenges.